1. Field of the Invention
This invention relates to atomising nozzles used for hand held sprayers such as so-called aerosols and pump type atomisers, intended for the application of liquid pharmaceutical products.
2. Description of the Related Art
Aerosol type sprayers are used throughout the world for dispensing a wide range of products, for example, hair lacquer, furniture polish, cleaners, paint, insect killers and medicaments. Until recently, chlorofluorocarbons (CFC's) were the most common of the propellant gases used in aerosols because they are inert, miscible with a wide range of products, are easily liquefied under low pressures, give a substantially constant product flow-rate, and can produce sprays of droplets having mean diameters in the range of 3 to over 100 micrometers. However, in the 1970's it was confirmed that CFC's were probably responsible for depleting the Earth's protective ozone layer, and in 1987, most countries signed the Montreal Protocol to phase out the use of CFC's and have since agreed to stop use of CFC's for non-essential applications by the end of 1995. One notable exemption to this deadline for cessation of use is in relation to metered dose inhalers (MDI's) for medicaments, which are regarded as an essential use of CFC's, but even this use of CFC's will eventually be phased out.
Gases such as air and nitrogen have the advantages of causing no environmental damage, being non-flammable and causing no ill effects if inhaled. Such gases can be used to propel liquid from a canister, but with a simple orifice or a swirl orifice very high pressures are required to produce a fine spray suitable for an MDI.
Other types of aerosol generators for delivery of liquid pharmaceutical products exist for research and hospital applications, such as nebulisers. However, these generally contain baffles to remove larger droplets and use high air flowrates so making them unsuitable for use in portable, convenient atomisers.
It is also possible to force liquid at high pressure through a very small hole (5-10 micrometers diameter) to produce droplets of about 5 micrometers diameter, but these methods are unsuitable or uneconomic for large scale manufacture, mainly because of the difficulty in making very small holes in a suitable material, and, to prevent blockage of the hole, the need for exceptional cleanliness in the manufacture of the parts, together with ultrafiltration of the fluid to be sprayed.
Many of the drugs used in the treatment of respiratory disorders are insoluble in vehicles such as water and are dispensed as suspensions. The drug particles are produced in a respirable size of 1-5 micrometers. Particles of this size tend to block the very small holes (5-10 micrometers) used by known devices.
For veterinary and some human vaccination applications, high pressure (125-500 bars) spring or gas operated pumps (so-called needle-less injectors) are in common use to inject a jet of drug through the skin ("intra-dermal injection") without the use of needles, and attachments are available to convert the jet to a spray for administering drugs to the nasal passages of large animals such as swine. However, the smallest droplet size obtainable is in the order of 40 micrometers, and the range of applications for these injectors is limited.
Compressed air atomisers such as air brushes and commercial paint sprayers consume large quantities of air, and to obtain droplets of 5 micrometers with water for example, a gas to liquid mass ratio of over 36:1 is required which is impractical for convenient, portable sprayers.
Spray nozzles in which a liquid is atomised by impingement of multiple jets of fluid on each other, e.g. air and liquid jets, are known. U.S. Pat. No. 5,385,304 describes an air assisted atomising spray nozzle in which a jet of liquid is atomised within a mixing chamber by the shearing action of several jets of air directed in substantially perpendicular relation to the liquid jet. The nozzle may be used to deliver liquid in a finely atomised state and suitable applications include use for the delivery of agricultural chemicals and pesticides, humidifying systems and scrubbing systems for coal furnaces. The nozzle described is believed to provide a high air efficiency by the use of an opposing cross-flow of air and the air/liquid mass ratio of the embodiment described is from 0.13 to 0.27. Although the spray particle size is not defined, the nozzle is described as producing a fine liquid droplet spray, and the applications discussed suggest that it might produce droplet sizes down to a minimum of 50 micrometers in diameter.
For MDI's used for treating certain respiratory disorders it is essential that the aerodynamic particle size should be less than 15 micrometers, preferably less than 10 micrometers, so that the droplets are able to penetrate and deposit in the tracheobronchial and alveolar regions of the lung. For a spray composed of droplets with a range of sizes, more than 5% by weight of the droplets should have an aerodynamic diameter less than 6.4 micrometers, preferably more than 20% by weight of the particles have an aerodynamic diameter less than 6.4 micrometers.
Inhalers may also be designed to deliver drugs to the alveolar sacs of the lung to provide a route for adsorption into the blood stream of drugs that are poorly adsorbed from the alimentary tract. To reach the alveoli it is essential that the aerodynamic diameter of the particles is less than 10 micrometers, preferably 0.5-5 micrometers
Current thinking suggests that to create smaller spray droplets from impinging fluid nozzles it is necessary to increase the gas/liquid mass ratio (GLR) resulting in an associated increase in gas reservoir size required to deliver the necessary mass of propellant. However, for the application of such technology to portable hand held inhalation devices, it is desirable for the GLR to be small to limit the size of reservoir required. The alternative of using hand or finger driven, or primed pumps to meter and produce the liquid and gas flows also requires that the volume and pressure of gas required are minimised to allow a small pump size and to minimise the effort required by the patient.